From DE 103 21 729 A, a force limiter is known that has a shaft and a metal strap, one end of which is attached to the shaft and which supports on a strap accommodation ring radially outside of the shaft. The force limiter serves to limit the force exerted by the safety belt on a vehicle occupant in a restraint case. This is achieved by the shaft of the force limiter being coupled to a belt retractor shaft that is non-rotatably connected to a belt reel. The strap accommodation ring in turn is non-rotatably connected to a housing or a frame of the belt retractor. Thus, the belt reel is secured to the frame via the belt retractor shaft, the shaft of the force limiter, the metal strap, and the strap accommodation ring. If in this state particularly high forces act in the safety belt, such a high torque is effective between the shaft of the force limiter and the strap accommodation ring that the metal strap passed several times around the inner wall of the strap accommodation ring is wound up on the shaft in a direction that is opposite to the direction in which the metal strap was originally incorporated in the strap accommodation ring. In so doing, the metal strap is bent twice, namely at first through approx. 180°, so that the metal strap extends in the opposite direction and is then bent back again somewhat so as to be in snug contact with the curvature of the shaft. This resistance to bending acts as a resistance torque between the strap accommodation ring and the shaft, this resistance torque representing in a known manner a limitation of the force being maximum effective in the safety belt.
In order to obtain with this known design a switchable characteristic curve for the force limitation, two of the energy converter coils described are provided as force limiters. These energy converter coils are connected in parallel in a first restraint phase so that a particularly high belt webbing force is required for achieving a relative rotation between the strap accommodation ring and the shaft of the force limiter. After the first phase one of the energy converter coils may be deactivated, so that then a lower belt webbing force is sufficient to obtain the aforementioned relative rotation. In this manner two levels for the force limitation may be achieved with the known design.
In contrast, the invention provides a force limiter that requires only one single energy converter coil, nonetheless permitting a force limitation that may selectively be changed during a restraint case.